A common query scenario that we have is something like this: "Select all contacts and their primary phone". For contacts with no phone (or no phone marked primary), we still want to get the contact back.

Before ANSI SQL, you had to do something like this:

SELECT c.Name, cp.Phone
FROM Contact AS c, ContactPhone as cp
WHERE cp.ContactId *= c.ContactId
AND ISNULL(cp.IsPrimary, 1) = 1

With ANSI SQL, this cleaned up to

SELECT c.Name, cp.Phone
FROM Contact AS c
LEFT OUTER JOIN ContactPhone AS cp
ON cp.ContactId = c.ContactId AND cp.IsPrimary = 1

Now, in HQL, since there's no "ON" clause for joins, it looks like we have to resort to using ISNULL again, or create an entity subclass for "primary phone". We will in fact create an entity subclass for this case, but there will be other cases conceived by the user at runtime.

Maybe I'm not understanding this right, but in the effort to eliminate "ON" clauses from HQL joins, it looks like some elegance was lost :?

Try:

That would only return contacts with primary phones. We want to return all contacts, whether they have a primary phone or not. Can HQL do this without using IsNull(), and without defining a new entity mapping for "primary phone"?

No, you can't do this easily with HQL currently, try using a native SQL query.

Or you may run two queries...

Resort to a native SQL query? :o :(

Using an HQL query with IsNull() would still seem to be much more preferable than resorting to a native SQL query. Native SQL queries through an NHibernate session can't return entities (or can they?)

Yes, they can. See http://www.hibernate.org/hib_docs/reference/en/html/querysql.html.